1. Field of the Invention
The present invention relates to an image sensor having self-aligned microlenses, and more particularly, an image sensor and a method for fabricating the same that maximizes an area of focusing lenses and absorbs a phase signal of unfocused light to prevent its photo-electrical conversion, thereby obtaining excellent photosensitivity.
2. Discussion of the Related Art
Resolution for an image sensors is determined by the number of photodiodes formed on the image plane that receives images. Accordingly, multi-pixel systems and miniaturization of a unit pixels are required. As the size of the unit pixel decreases, so does the area of a light-receiving portion. This requires focusing external images onto the image plane. To enhance photosensitivity, i.e., to increase the levels of sensed light, a focusing lens is used. The focusing lens is formed above or below a color filter layer formed as an array of color filters. To obtain a clear image it is necessary to have more light in focus than out of focus. Since the degree of focus of the incident light depends on the size of the sectional area of the focusing lens, clearer images are obtainable by maximizing the sectional area of focusing lens.
With the miniaturization and the multi-pixel system of the image sensor, more pixels per sectional area are formed in an on-chip type configuration. As pixel size decreases, the sizes of the filters of the color filter layer and the microlenses of a microlens layer also become smaller. As the pixel size becomes small, a photodiode area that receives light is also reduced, thereby reducing the photosensitivity. To compensate for the reduced photosensitivity, the photodiode area requires more light. To this end, a method of increasing an opening size and a method of forming focusing microlenses are provided. The openings are formed for the focusing lenses. These openings are created by forming a metal layer that serves as a wiring and as a light-shielding layer. In this manner, the light incident to the light-shielding layer is refracted toward the openings. Adhesion of the lenses varies as the size of focusing lenses is maximized. This causes image uniformity characteristics to suffer. The focusing of the lenses is also affected by the color filter layer, specifically, if the image signals of adjacent color filters are mixed they degrade the color information and contrast in the reproduced image. Additionally, it is difficult to form a precise pattern during alignment exposure due to the poor optical resolution caused by a mixture of pigment when the individual color filters of the color filter layer are formed. A planarization layer is also required because of the overlapping and gaps that occur between the color filters.